Satellite radio, quite simply, is a non-terrestrial microwave transmission system utilizing a space relay station. Satellites have proved invaluable in extending the reach of voice, data, and video communications around the globe and into the most remote regions of the world. Exotic applications such as the Global Positioning System (GPS) would have been unthinkable without the benefit of satellites.

Contemporary satellite communications systems involve a satellite relay station that is launched into a geostationary, geosynchronous, or geostatic orbit. Such satellites are called geostationary satellites. Such an orbit is approximately 36,000 km above the equator as depicted in Figure. At that altitude and in an equatorial orbital slot, the satellite revolves around the earth with the same speed as of that the speed of revolution of earth and maintains its relative position over the same spot of the earth's surface. Consequently, transmit and receive earth stations can be pointed reliably at the satellite for communications purposes.

The popularity of satellite communications has placed great demands on the international regulators to manage and allocate available frequencies, as well as the limited number of orbital slots available for satellite positioning are managed at national,regioi1al and international levels. Generally speaking, geostationary satellites are positioned approximately 2° apart in order to minimize interference from adjacent satellites using overlapping frequencies.

Such high frequency signals are especially susceptible to attenuation in the atmosphere. Therefore, in case of satellite communication two different frequencies are used as carrier frequencies to avoid interference between incoming and outgoing signals. These are:

Uplink frequency It is the frequency used to transmit signal from earth station to satellite. The uplink signal can be made stronger to cope better with atmospheric distortion. The antenna at transmitting side is centered in a concave, reflective dish that serves to focus the radio beam, with maximum effect, on the receiving satellite antenna. The receiving antenna, similarly, is centered in a concave metal dish, which serves to collect the maximum amount of incoming signal.

Downlink frequency It is the frequency used to transmit the signal from satellite to earth station. In other words, the downlink transmission is focused on a particular footprint, or area of coverage. The lower frequency, used for the downlink; can better penetrate the earth's atmosphere and electromagnetic field, which can act to bend the incoming signal much as light bends when entering a pool of water.

Broadcast The wide footprint of a satellite radio system allows a signal to be broadcast over a wide range Thereby; any number (theoretically an infinite number) of terrestrial antennae can receive the signal, more or less simultaneously. In this manner, satellites can serve a point-to-multipoint network requirement through a single uplink station and multiple downlink stations,

Recently, satellites have been developed which can serve a mesh network, requirement, whereby each terrestrial site can communicate directly with any other site. Previously, all such communications were required to travel through a centralized site, known as a head end. Such a mesh network, of course, imposes an additional level of difficulty on the network in terms of management of the flow and direction of traffic.

General Properties of Satellite Communication

• Configuration Satellite communication systems consist of antennae and reflective dishes, much as a terrestrial microwave. The dish serves to focus the signal from a transmitting antenna to a receiving antenna. The send (receive) dishes that make up the earth segment are of varying sizes, depending on power levels and frequency bands. They generally are mounted on a tripod or other type of brace, which is anchored to the earth, pad or roof or attached to a structure such building. Cables connect the antennae to the actual transmit (receive) equipment. The terrestrial antennae support a single frequency band, for example, C-band, Ku-band or Ki-band. The higher the frequency bands the smaller the possible size of the dish. Therefore, while C-band TV dishes tend to be rather large, Ku-band DBS (Direct Broadcast Satellite) TV dishes tend to be very small. The space segment dishes are mounted on a satellite, of course. The satellite can support multiple transmit (receive) dishes, depending on the various frequencies which it employs to support various applications, and depending on whether it covers an entire footprint or divides the footprint into smaller areas of coverage through the use of more tightly focused spot beams· Satellite repeaters are in the form of number of transponders. The transponders accept the weak incoming signals, boost them, shift from the uplink to the downlink frequencies, and transmit the information to the earth stations.

• Security is with all microwave and other radio systems, satellite transmission is inherently not secure. Satellite transmission is especially vulnerable to interception, as the signal is broadcast over the entire area of the footprint. Therefore, the unauthorized user must know only the satellite and associated frequency range being employed. Security must be imposed through encryption (scrambling) of the signal.

• Cost The acquisition, deployment, and rearrangement costs of the space segment of satellite systems can be quite high in several million dollars. However, the satellite can be shared by a large number of users, With each user perhaps connecting a large number of sites. As a result, satellite networks often compare very favorably with cabled systems or microwave systems for many point-to-multipoint applications.

• Application Satellite applications are many and increasing rapidly as the traditional voice and data services have been augmented Traditional international voice and data services have been supplanted to a considerable extent by submarine fiber optic' cable system.